I was using Tomato firmware for months without any problem on a E3000 using a Zyxel P-660RU-T1 as a modem/brdige for ADSL2, with the default MTU 0f 1492 set on the WAN interface using pppoe. However, one week ago I changed my modem/bridge for a Zyxel P-870H which supports VDSL2. The only parameters I have changed are the MTU set to 1462 (provided by the ISP) and the DSL Overhead Value - ATM Encapsulation Type which as been set to none. But as soon as I changed those parameters, my ps3 has started complaining about "Router in use may not support IP fragments…". However, if I connect direct to the Zyxel and configure it as a router (with NAT), I don't have this message anymore. Why do I have this message when connecting from my E3000 with the Zyxel as a modem/bridge ?

You give very little info to work with, and not many people use DSL. Why did you change the two parameters you mentioned ? Will the router not work with the new modem using the old parameters ? What were the old parameters ? Did you try leaving the new modem in gateway/nat mode and running that into your WAN port for a dose of double NAT ? In the absence of good answers to my first two long shot questions I would try double NAT. Leave the router at factory defaults for the double NAT test.

I know I haven't given a lot of information, but I may try to give good answers to your questions. I have changed the MTU parameter because it has been given by my new ISP (1462 was the default on my router). The second parameter (DSL Overhead Value - ATM Encapsulation Type) has been changed because it applies only for ADSL and not for VDSL2. VDSL2 interface supports Packet Transfer Mode (PTM) and PTM is based on the Ethernet in the First Mile (EFM) IEEE802.3ah standard. However, to answer your other questions, I have tried to keep the old MTU parameter and the old DSL Overhead Value - ATM Encapsulation Type parameter, but it doesn't change anything. By the way, I have tried to completly disable Qos, but the problem is the same. But I haven't tried the double NAT configuration. What may be the (dis)advantage to have double NAT and two routers ? Anyway, I'm going to give a test to that one as it is the only advice I had for now.

I have previously played with MTU for different ISP connections and would sometimes see a little improvement with certain settings. Nothing that was consequential. I think if the router MTU is larger than the ISP MTU then bandwidth was maximum. I am not familiar with the second setting that is DLS specific, but since you tried it both ways with no difference in operation it does not sound very critical either.

The double NAT should cause no problems. The question comes up in these forums from time to time and somebody usually mumbles something about breaking an old VOIP protocol, but I do not think there is much to the story these days. For your double NAT test, I would first try factory default settings on the Tomato router with perhaps a change of IP address on the LAN if needed, but all other settings at default.

Ok, I have tried to put the tomato E3000 router as the DMZ host of the Zyxel P-870 with double NAT. It is only working partially because some services are working fine, but my Gigaset IP phone is not able to authenticate anymore with my SIP provider. As my Gigaset C610 IP phone is not so old, as it anything to do with an old VOIP protocol or does the tomato router unfriendly NAT transversal for VOIP ? Anyway, as long as my IP phone does not work as expected, it is not the definitive solution for me.

Yes, it is exactly what I have done ant it works well. However, if I'm forced to bypass Tomato router and qos for the IP phone, it doesn't make sense for me to keep qos on Tomato. So I'll have to take a look on the qos stuff on the Zyxel, which I have discovered, is running with MIPS Linux kernel version 2.6.30, BusyBox 1.0.0 toolset, iptables, ppp (from roaring penguin pppoe) and so on. This makes me feel like it would be possible to run a version of Tomato that works fine as does the Zyxel router over my xDSL line using those tools.